The mechanism known formally as the Trammel of Archimedes (Wikipedia) has practical application, when fitted with a drawing or cutting tool, as an ellipsograph. Otherwise it is fascinating but generally useless, and these qualities have earned it a variety of approximately affectionate epithets: “do nothing machine,” “nothing grinder,” “BS grinder,” and “Kentucky do-nothing” are a few I’ve heard. Recently, I was thinking about building one as a gift, and found this great tutorial by Instructables user perry112358.

My 10 favorite mechanical animations from Wikipedia